1. Field of the Invention
The present invention relates in general to magnetic resonance (MR) tomography as used in medicine for examining patients. The present invention relates in particular to an MR tomography apparatus of the type wherein vibrations of apparatus components which negatively influence many aspects of the overall system are reduced.
2. Description of the Prior Art
MR tomography is based on the physical phenomenon of nuclear spin resonance and has been used successfully as an imaging method for over 15 years in medicine and in biophysics. In this method of examination, the object is exposed to a strong, constant magnetic field. This aligns the nuclear spins of the atoms in the object, which were previously oriented irregularly. Radio-frequency energy can now excite these xe2x80x9corderedxe2x80x9d nuclear spins to a specific oscillation. In MR tomography, this oscillation generates the actual measurement signal which is picked up by means of suitable receiving coils. By the magnetic fields which are not spatially constant, generated by gradient coils, it is possible to spatially code the measurement signals from the object in all three spatial directions. The method permits a free choice of the layer to be imaged, so that it is possible to obtain tomographic images of the human body in all directions. MR tomography in medical diagnostics is distinguished first and foremost as a xe2x80x9cnon-invasivexe2x80x9d method of examination by a versatile contrast capability. MR tomography currently uses sequences with high gradient performance which permit an excellent image quality with measuring times of the order of magnitude of seconds and minutes.
Continuous technical development of the components of MR systems, and the introduction of high-speed imaging sequences have opened ever more fields of use for MR tomography in medicine. Real time imaging for supporting minimally invasive surgery, functional imaging in neurology and perfusion measurement in cardiology are only a few examples.
The basic design of the examination portion of such an MR apparatus is illustrated in FIG. 4. This has a basic field magnet 1 (for example an axial superconducting air-coil magnet with active stray field shielding) which generates a homogeneous magnetic basic field its interior volume. The superconducting magnet 1 has superconducting coils which are contained in a vessel of liquid helium. The basic field magnet 1 is surrounded by a two-shell vessel arrangement which is made from stainless steel, as a rule. The inner vessel, which contains the liquid helium, and serves in part also as a winding body for the magnet coils, is suspended at the outer vessel, which is at room temperature, via fiber-glass-reinforced plastic rods which are poor conductors of heat. A vacuum prevails between inner and outer vessels.
The cylindrical gradient coil arrangement 2 in the interior of the basic field magnet 1 is inserted concentrically into the interior of a support tube by means of support elements 7. The support tube is delimited externally by an outer shell 8, and internally by an inner shell 9. The function of the layer 10 will be explained later.
The gradient coil arrangement 2 has three such windings which, respectively generate gradient fields, proportional to the current impressed in each case, and which are spatially perpendicular to one another. As illustrated in FIG. 5, the gradient coil 2 has an x-coil 3, a y-coil 4 and a z-coil 5, which are respectively wound around the coil core 6 and thus generate respective gradient fields in the directions of the Cartesian coordinates x, y and z. Each of these coils is provided with a dedicated power supply unit in order to generate independent current pulses with accurate amplitudes and timing in accordance with the sequence programmed in the pulse sequence controller. The required currents are approximately 250 A. Since the gradient switching time is to be as short as possible, current rise rates of the order of magnitude of 250 kA/s are necessary. In an exceptionally strong magnetic field such as is generated by the basic field magnet 1 (typically between 0.22 and 1.5 tesla), such switching operations are associated with strong mechanical vibrations because of the Lorentz forces that occur. All system components (housing, covers, vessels of the basic field magnet and magnet housing, respectively, RF body coil etc.) are excited to produce forced vibrations.
Since the gradient coil is generally surrounded by conductive structures (for example magnet vessel made from stainless steel), the pulsed fields start in eddy currents in such structures, which exert forces on these structures due to interaction with the basic magnetic field, and likewise excite these structures to vibrations.
There is one further source of vibration which chiefly excites the magnet vessel to vibrations. This is the so-called cold head 15, which ensures that the temperature of the basic field magnet 1 is maintained. The cold head 15 is driven by a compressor and exerts mechanical forces on the housing of the basic field magnet 1.
These vibrations of the various MR components act negatively in many ways on the MR system:
1. Decidedly stronger airborne noise is produced, which constitutes an annoyance to the patient, the operating staff and other persons in the vicinity of the MR system.
2. Vibrations of the gradient coil and of the basic field magnet, and their transmission to the RF resonator and the patient couch in the interior of the basic field magnet and/or the gradient coil, are expressed in inadequate clinical image quality which can even lead to misdiagnosis (for example in the case of functional imaging, fMRI).
3. High costs arise also due to the need for a vibration-damping system setupxe2x80x94similar to that of an optical tablexe2x80x94in order to prevent transmission of the vibrations to the ground, or vice versa.
In known systems, the transmission of vibrational energy between the gradient coil and tomograph is countered by the use of mechanical and/or electromechanical vibration dampers.
German Offenlegungsschrift 197 22 481 discloses an MR tomography apparatus which has a magnet body surrounded by a magnet housing, and this magnet housing surrounds and delimits an interior. A gradient coil system is located in this interior. A damping structure is provided on an inner side, delimiting the interior, of the magnet housing for absorbing acoustic vibrations which are caused by switching the currents generating the gradient fields. At least one damping cushion is formed of liquid, gaseous or flowable materials.
German Offenlegungsschrift 197 34 138 likewise discloses the use of passively acting elastomeric damping elements (for example rubber bearings), as well as the encapsulation of the gradient coil in a separate vacuum housing.
Normally use is made as well of the following further passive measures in order to reduce the vibrations:
general encapsulation of the source of vibration
use of thicker and heavier materials
damping layers (for example tar) applied from xe2x80x9coutsidexe2x80x9d.
In particular, the noise production path over the interior of the MR apparatus, i.e., production of noise by vibration of the gradient coil and transmission of the noise to the support tube located in the gradient coil (FIG. 4), which emits the noise inwardly to the patient and the interior, is blocked in the system disclosed in U.S. Pat. No. 4,954,781 by a damping viscoelastic layer 10 (FIG. 4) in the double-ply inner layer 9 of the support tube.
Furthermore, it is known to achieve this by inserting sound-absorbing so-called acoustic foams into the region between support tube and gradient coil.
German Offenlegungsschrift 196 43 116 discloses the integration of magnetostrictive materials, in particular into the gradient coil, permitting active, controlled opposing vibration-countering during operation, thereby reducing the vibration amplitude of the gradient coil.
German Patentschrift DE 44 32 747 likewise teaches the use of active damping, but by means of integrated electrostrictive piezoelectric actuators.
Nevertheless, the acoustic emission of a conventional apparatus continues to be very high.
It is an object of the present invention to further reduce the noise transmission during operation of an MR apparatus.
This object is achieved according to the invention in an MR tomography apparatus having a magnet body surrounded by a magnet housing which surrounds and delimits an interior volume, with a gradient coil system located in this interior volume. Provided on an inner side, delimiting the interior, of the magnet housing is a damping laminated sheet structure for absorbing acoustic vibrations which are produced by switching of the gradient coil system.
The annoyance represented as noise is advantageously reduced thereby. Moreover, the transmission of vibrations to the RF resonator and the patient couch is reduced so that the clinical image quality is not substantially impaired.
There are various possibilities for configuring the design of the damping laminated sheet structure:
If the damping laminated sheet structure constitutes an open system, an inner sheet forms the vacuum-bearing inner wall of the magnet housing, and the actually damping outer wall of the magnet housing is formed by an outer sheet together with a damping plane lying between the two sheets.
The open system advantageously extends in this case only over the partial surface of the magnet housing which faces the interior.
If the damping laminated sheet structure constitutes a closed system, both the inner sheet and the outer sheet form the vacuum-containing wall of the magnet housing, a damping plane being located between the two sheets.
The closed system then advantageously extends only over the partial surface of the magnet housing which faces the interior.
However, it can also be advantageous for the closed system to extend over the entire surface of the magnet housing.
The damping laminated sheet structure is advantageously formed from two sheets together with a damping plane located therebetween.
However, it can also be advantageous to form the damping laminated sheet structure in a multilayer design as a closed system composed of a number of sheets together with damping planes lying therebetween.
In this case, the damping plane is advantageously a viscoelastic layer.
In order to prevent transmission of the mechanical vibrations of the cold head to the magnet housing, in a further embodiment of the present invention the damping laminated sheet structure is laid in annular fashion around the cold head in the form of an open or closed system.